When a muscle needs to be moved, a nerve impulse is created by the central nervous system and sent down a motor neuron toward the muscle. The impulse causes calcium channels in the membrane of the neuron to open, which creates an inrush of Ca+2 ions into the terminal end of the neuron. The incoming calcium causes vesicles of acetylcholine to fuse with the neuron's cell membrane, releasing acetylcholine into the synapse, or gap, between the neuron and the motor end plate of the muscle cell. The acetylcholine crosses the synapse and activates sodium/potassium channels in the motor end plate, which allow sodium to enter the muscle cell while potassium is allowed to leave. This depolarizes the muscle cell.
The depolarization causes a series of actions resulting in myosin in the muscle cell binding to actin;the myosin binds by using the energy from an ATP molecule. The muscle fiber releases ADP and a phosphate ion and moves through the power stroke, which causes the muscle fiber to contract. Then another ATP binds the myosin, causing it to release the actin, and the fiber relaxes and is ready for the next stroke.